Disc drive memory systems store significant amounts of digital data that is recorded on a relatively small area of concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to store and retrieve information from the magnetic discs. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the stator. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the sleeve, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
Over the years, storage density has increased, and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities, the read/write heads must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the read/write heads, possibly damaging the disc drive and resulting in loss of data. This is known as a “crash” and can damage the read/write heads and surface of the storage disc, resulting in loss of data. This out-of-flat condition can also create a variation in spacing between the head and disc during reading or writing operations, which also creates a reduction in data integrity.
Spindle motors have in the past used conventional ball bearings between the sleeve and the shaft. However, the demand for increased storage capacity and smaller disc drives has led to the design of higher recording area density such that the read/write heads are placed increasingly closer to the disc surface. Because satisfactory rotational accuracy cannot be achieved using ball bearings, disc drives currently utilize a fluid dynamic bearing between a shaft and sleeve to support a hub and the disc for rotation. An alternative bearing design is a hydrodynamic bearing.
In a hydrodynamic bearing, a lubricating fluid such as gas or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. Hydrodynamic bearings spread the bearing surface over a large surface area, as opposed to a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members, decreasing fragility and improving integrity of the motor. Further, the use of fluid in the interface area imparts damping effects to the bearing, which helps to reduce non-repeat run-out. The hydrodynamic bearing motor design is desirable for its improved angular stiffness and dynamic performance; however, manufacturing of motor components can be costly. Stringent performance requirements on current hydrodynamic bearing motor designs require tight component tolerances, often necessitating expensive secondary machining of components.